This invention relates to a device and method for cooling a fuel cell stack of a fuel cell system.
A fuel cell generates electrical energy through electro-chemical reaction of hydrogen and oxygen. The fuel cell system is generally provided with a cooling device. The cooling device maintains a fuel cell stack of the fuel cell system to a substantially predetermined temperature when the fuel cell stack generates heat as a result of the electro-chemical reactions. For example, in a fuel cell system comprising a polymer electrolyte fuel cell (PEFC) stack, the cooling device maintains a temperature of 80xc2x0 C. while the fuel cell stack is operating. The cooling device has a recirculation system for a cooling liquid. The cooling liquid in the recirculation system is supplied to the fuel cell stack by a recirculation pump. After passing through the fuel cell, the cooling liquid extracts resultant heat with a heat exchanger such as a radiator. Thereafter the cooling liquid is returned to a tank.
Highly pure water is generally used as the cooling liquid. If the water displays high electrical conductivity, there is the possibility that the fuel cell may develop a short circuit. Short circuits in the fuel cell can reduce the amount of power generation or stop power generation entirely. A conductivity reduction device such as an ion removal filter is provided in the cooling liquid system in order to reduce the conductivity of the cooling liquid. Tokkai Hei 9-22716 published by the Japanese Patent Office in 1997 discloses such an ion removal filter. When the conductivity of the cooling liquid in this prior-art device reaches a permitted limiting value, an ion removal process is performed in order to reduce conductivity of the cooling water.
However the prior-art ion removal filter can not operate when the cooling device displays maximum cooling performance or when there is not a margin in power supply to the recirculation pump, even if the cooling liquid has reached a permitted limiting value. This is because cooling must be given priority or that there is insufficient excess power to drive the pump transferring cooling water to the ion removal filter. This problem tends to occur particularly in vehicle fuel cell system because it is difficult to ensure a high margin of power in a vehicle fuel cell system.
It is therefore an object of this invention to increase the operating efficiency of a conductivity reduction device of the cooling liquid that cools a fuel cell stack of a fuel cell system.
In order to achieve above object, this invention provides a cooling device for a fuel cell stack comprising a heat exchanger which cools the cooling medium, a cooling passage through which the cooling medium is circulated between the fuel cell stack and the heat exchanger, a pump which pressurize the cooling medium in the cooling passage for circulation, a bypass passage which bypasses the fuel cell stack to directly circulate a part of the cooling medium discharged from the pump to the heat exchanger, an electrical conductivity reducing mechanism which reduces electrical conductivity of the cooling medium passing through the bypass passage, a valve which regulates a bypass amount of the cooling medium passing through the bypass passage, a sensor which detects an electrical conductivity of the cooling medium and a programmable controller.
The controller is programmed to determine if the electrical conductivity is larger than a predetermined conductivity, control the valve to increase the bypass amount when the electrical conductivity is larger than the predetermined conductivity, determine if a predetermined condition related to an operation of the fuel cell stack holds, and control the valve to prevent the bypass amount from becoming zero when the electrical conductivity is not larger than the predetermined conductivity and the predetermined operation condition holds.
This invention also provides a control method for a cooling device which cools a fuel cell stack of a fuel cell system. The device comprises a heat exchanger which cools the cooling medium, a cooling passage through which the cooling medium is circulated between the fuel cell stack and the heat exchanger, a pump which pressurize the cooling medium in the cooling passage for circulation, a bypass passage which bypasses the fuel cell stack to directly circulate a part of the cooling medium discharged from the pump to the heat exchange, an electrical conductivity reducing mechanism which reduces electrical conductivity of the cooling medium passing through the bypass passage, a valve which regulates a bypass amount of the cooling medium passing through the bypass passage. The method comprises detecting an electrical conductivity of the cooling medium, determining if the electrical conductivity is larger than a predetermined conductivity, controlling the valve to increase the bypass amount when the electrical conductivity is larger than the predetermined conductivity, determining if a predetermined condition related to an operation of the fuel cell stack holds, and controlling the valve to prevent the bypass amount from becoming zero when the electrical conductivity is not larger than the predetermined conductivity and the predetermined operation condition holds.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.